The invention relates to sandwich structures having curved core and a forming process wherein the structure is formed from flat pieces.
Sandwich structures are characterized by core structure positioned between two face sheets. As used herein, the term "sandwich structures" includes beams. Sandwich structures have attained widespread use in advanced aircraft, particularly in wings, wall panels, doors, webs of beams, and the like. These lightweight structures can withstand large normal loads. Considerable research and development has resulted in novel tooling and production methods to create these unique structures. Many of the materials used have superplastic properties.
"Superplasticity" refers to the property of certain materials, particularly metal alloys, to exhibit unusually high tensile elongations with reduced tendency towards necking. The elongations occur when the microstructure is properly heated to within a predetermined temperature range and when the structure is deformed within a predetermined stress range. Many materials may be used; including but not limited to aluminum, titanium, copper, and their respective alloys, as well as plastics, composites and steel. The titanium alloy, Ti-6Al-4V, has generally been preferred in sandwich structures for a number of reasons, including the adaptability of the alloy to diffusion bonding.
Since sandwich structures consist of a number of workpieces, the workpieces must be joined together to form a finished structure. The term "joining" as used herein refers to any method of attaching two workpieces together. The joining can be accomplished by brazing, welding, bonding, or by applying adhesives. "Welding" processes include cold welding, fusion welding, pressure welding, and laser welding, and involve the metallurgical joining of the surfaces by applying heat, causing the materials at the joint interface to reach a liquid or plastic state and merge into a unified whole. "Bonding" processes include diffusion bonding, deformation bonding, and solid state bonding and involve metallurgical bonding wherein the material in the bond is similar in composition and state to the surrounding material. "Diffusion bonding" as used herein refers to the solid state joining of similar or dissimilar metals by applying heat and pressure for a time duration to cause commingling of the atoms at the joint interface.
Since temperature and differential pressure ranges for superplastic forming and diffusion bonding are similar in many applications, the processes may be used together in what is essentially a one-step operation. However, a disadvantage of superplastic forming is that it requires the use of superplastic materials raised to superplastic forming temperatures and pressures.
U.S. Pat. No. 4,361,262 entitled "Method of Making Expanded Sandwich Structures" to L. Israeli, which is incorporated by reference into this specification, discloses a process that may be used as a substitute for superplastic forming. The process is called "accordion expansion" and essentially involves the unfolding of core sheets with a minimal amount of expanding. The formed sandwich structures generally have a vertical core that is linear and flat, although angled core is also possible. To form such structures only about 10% expansion is required. Since almost all metals will expand 10% without a significant loss of strength at elevated temperatures, accordion expansion is not limited to superplastic materials.
Oftentimes, sandwich structures having flat core do not provide sufficient strength to normal loads. It has been estimated that sandwich structures having a sine wave core configuration are about 150% stronger than such structures having a flat core. In addition, other curved core configurations can be used in specific applications where linear core is inadequate. A major advantage of accordion expansion is that forming can occur at temperatures and pressure differentials significantly lower than superplastic forming. Clearly, what is needed is a method of making stronger sandwich structure configurations that is not limited to superplastic materials.